Color construction of multi-colored carbon fibers using glucose

Carbon fibers (CFs) have attracted attention in the automotive, aviation, and aerospace industries. However, the coloration of CFs is challenging due to their brittleness, inertness, complexity, and time/energy-intensive processes. Herein, inspired by the naturally grown protrusive nanostructures on the green central surface of peacock back feathers, we report an in-situ self-growing strategy for developing carbon spheres (CSs) on the CFs surface to achieve color tuning. This is achieved via the dynamic growth of CSs using glucose as the feeding material. Combined with the coloration process, the interaction between CSs and CFs promotes stable interfacial forces in integrated molding. This strategy allows the coloring system to continuously vary its color in a designated manner, thereby, endowing it with satisfactory mechanical robustness, acid durability, and light fastness. We anticipate this developed approach can be potentially competitive in the color construction of CFs with multi-colors due to its low-cost manufacturing.

As shown in Supplementary Fig. 2, photographs of the different colored CF fabrics, derived from glucose reaction concentrations of 5CC, 6CC, 8CC, 9CC, 11CC, 12CC, 14CC, 15CC, and 16CC, were presented by the maximum absorption wavelength of the UV diffuse absorption spectrum to characterize the difference between similar colors.Supplementary Fig. 3 K/S spectra of 4CC, 7CC, 10CC, 13CC, and 17CC.K/S value is a function (K/S=(1-R) 2 /2R) developed by Kubelka and Munk, who theorized in 1931 that the ratio of the coefficient of light absorption (K) to the coefficient of light scattering (S) is corresponding to the fractional reflectance of the light (R) of the opaque substrate at a given wavelength 1,2 .Here, K/S values of the colored CF fabrics could be directly measured by spectrophotometer at λmax, which was presented with K/S=(1-Rλmax) 2 /2Rλmax.Therefore, the wavelength corresponding to the maximum K/S value (Supplementary Fig. 3), matching with the characteristic wavelengths of reflectance spectra, represented the color strength used to assess the stability of color strength after different treatments.The CSs growth process is investigated by treating the same concentration of glucose with four reaction stages of 30, 60, 90, and 120 min at 250 °C.The colors of reaction liquids were turned from yellow to brownish black with the increase of reaction time (Supplementary Figs. 10 a-d).As shown in Supplementary Fig. 10, SEM images are used to investigate the effects of reaction time on the CSs formation process.At 120 min of hydrothermal reaction (Supplementary Fig. 10 d), a large number of CSs could be observed on the surface of the CFs.These results suggest that there is no aggregation of particles during the initial stage of the reaction, and CSs form gradually on the surface of CF with the time extension in the process of hydrothermal reaction.X-ray photoelectron spectroscopy (XPS) was performed to investigate the functional groups of raw glucose and the solid products after hydrothermal reaction with reaction times of 30, 60, 90, and 120 min at 250 °C.As shown in Supplementary Fig. 11, after the hydrothermal reaction, the surface C/O ratio of the solid significantly increased, which is different from the raw glucose (Supplementary Fig. 11 a).Compared to the raw glucose (50.93%), the surface carbon content of solid product increased to 73.97% within 30 min of the reaction.In the subsequent reaction process, the carbon content was maintained in a range of 71.08%-75.33% at different reaction times.In the C1s spectra, C-C/C=C, C-O, C=O, and O-C=O are assigned to 285.0, 286.3, 287.5, and 288.9 eV, respectively 3 .The significant changes of the C1s XPS has been displayed with the forming of C=C, O-C=O, and C=O bonds via the hydrothermal reaction, which is different from the raw glucose (Figs. 2 f-i and Supplementary Fig. 11).The presence of these oxygenated groups was confirmed by the O1s spectrum.The signals of O1s spectra attribute to the O-C (531.0 eV), and O=C (532.1 eV) 4 , which confirm the formation of oxygenated groups O=C bonds during the hydrothermal reaction 5 .The oxygen-containing functional groups present on the surface of solid products may consist of more reactive/ hydrophilic groups 6 .Combining with the photographs of products (Supplementary Fig. 10) and SEM images at different reaction stages, Supplementary Fig. 11 shows the changes of functional group for the products at the various reaction stages.Supplementary Fig. 12  Supplementary Fig. 12 shows the 110-150 ppm region during the hydrothermal reaction of glucose at 250 °C with the extension of time.In the early stages of the reaction, the region is characterized by the presence of two peaks, which are due to the furanic rings (140-153 and 110-120 ppm).At 30 min, the products obtained from glucose of hydrothermal have a polymer-like structure composed of polyfuranic chains domains 7 , along with the practical absence of a central peak at 125-129 ppm (Supplementary Fig. 12 a).As the HTC residence time increases, the relative intensity of the central peak at 125-129 ppm starts forming (Supplementary Fig. 12 b) with the production of solids (Supplementary Fig. 10 c).Corresponding to the SEM images, the NMR spectra show the relative intensity of the peaks at 140−153 ppm, and 110−120 ppm are assigned to the carbons of Cα and Cβ assigned to the furanic ring.With the extension of reaction time, the obvious intensity of the peak at 125-129 ppm is observed, which can be assigned to carbon atoms belonging to aromatic rings, demonstrating the aromatization of the polyfuranic compounds.Compared to the solid 13 C NMR spectra of 90 min, the relative intensity of the peak at 125-129 ppm enhanced in Supplementary Fig. 12 d, demonstrating the increased degree of aromatization.As shown in Supplementary Figs.10-12, due to the intramolecular dehydration, condensation, and decarboxylation, the more condensed sp 2 hybridized-aromatic chemical species was created 8 .Nucleation then occurs at the critical supersaturation point of insoluble clusters.Furthermore, the active groups of the surface promote the growth and formation of CSs with the enhancement of hydrothermal carbonization 6,9 .Supplementary Fig. 15 Photographs of the fabricated CF fabrics before and after mechanical rubbing test performed by color fastness friction meter with a load pressure of 50 kPa for 10 cycles, the soaking and washing of acetic acid solution test after mechanical rubbing test at a vibration of 60 times per minute for 120 min, the accelerated light-aging test under the strong light using a xenon lamp light source system for simulated sunlight after mechanical rubbing test and soaking and washing of acetic acid solution, with the environment temperature, humidity, and light irradiance were set as 38 °C, 47 % RH, and 1.271×10 4 W m -2 , for 4CC, 7CC, 10CC, 13CC, and 17CC, respectively.The reaction temperature offers two fundamental functions, providing heat for dissociation to accelerate the polymerization and aromatization reactions 10 .Meanwhile, the residence time determines the extent of carbonization at a given temperature 11 .The evolution of the carbon-related chemical structures in the HTC process was affected by the combined influence of temperature and residence time, which maintained constant during the reactions.Supplementary Table 1 shows that the value of carbon content was 67.17 % under the reaction temperature of 250 °C and time of 2 h, thus producing a carbon-rich CSs 12,13 .

Supplementary
Supplementary Table 2 Different methods of preparing structurally colored CFs.

Supplementary Fig. 2
Photographs and the corresponding absorption spectra of various CF fabrics with different glucose solution concentrations.Photographs and the corresponding absorption spectra of a 5CC, b 6CC, c 8CC, d 9CC, e 11CC, f 12CC, g 14CC, h 15CC, i 16CC, respectively.

Supplementary Fig. 10
Photographs of the reaction products and SEM images at various reaction times.Photographs of reaction products and SEM images reaction products carbon fibers (CFs) at a 30 min, b 60 min, c 90 min, and d 120 min in the HTC reaction process with glucose of 7 g 70 mL -1 .

Supplementary Fig. 11 XPS
survey spectra.a XPS survey of C1s, and O1s spectra of glucose.XPS survey spectra and O1s spectra after hydrothermal reaction of glucose at different stages b 30 min, c 60 min, d 90 min, and e 120 min at 250 °C.
NMR spectra of as-prepared samples at 250 °C with the extension of time.Solution 13 C NMR spectra of a 30 min and b 60 min after hydrothermal reaction of glucose at 250 °C.Solid 13 C NMR spectra of c 90 min and d 120 min after hydrothermal reaction of glucose at 250 °C.
Photonic crystal Particle sizes

Table 1
Carbon content of hydrochars derived by glucose.

Table 3
The K/S value at three different positions of colored CF fabric (R), after mechanical rubbing test (T), acid pickling (A), and accelerated light-aging test (L) for colored CF fabric, successively.

Table 4
Average K/S values after mechanical rubbing.Tx; T: mechanical rubbing test; x: the cycle times.